United States laws and regulations obligate an oil filler pipe for an automobile to assure a life of 15 years or 150,000 miles running. An oil filler pipe made of stainless steel (SUS436L: 17Cr-1.2Mo) has been already put into practical use.
An automobile traveling in North America or Europe is exposed to an environment with snow-melting salt, so that a material used for an oil filler pipe needs to have an excellent salt corrosion resistance. Accordingly, SUS436L has been used as a material for an oil filler pipe. However, cost reductions have come to be demanded due to the recent increase in resource prices. To achieve cost reductions, integrally molded components may be used to reduce the number of components instead of using an inexpensive material. In this case, a metal fitting should be directly attached to an inlet pipe as a main component. This entails formation of a gap structure between the inlet pipe and the attached component. Corrosion is likely to occur in a gap as compared with a ‘general part’ other than the gap. In particular, crevice corrosion occurs in a gap on an outer surface of an oil filler pipe exposed to a chloride environment. If the inlet pipe, through which fuel passes, is holed due to crevice corrosion, fuel leakage may occur, causing a serious accident.
Conventionally, coating such as cation electrodeposition coating has been applied as a means for improving salt corrosion resistance in the gap.
For example, Patent Literature 1 discloses a production method of subjecting an oil filler pipe assembled by projection welding using a SUS436 pipe as a material to cation electrodeposition coating. However, this technology, which uses SUS436 as a material, cannot completely prevent corrosion because it is difficult to apply cation electrodeposition coating to a gap present on a surface of the oil filler pipe even when SUS436 is used, according to the knowledge of the inventors. Accordingly, this technology is supposed to be unable to offer a sufficient rust prevention effect in an environment with snow-melting salt.
Patent Literature 2 discloses a technology of preventing crevice corrosion by subjecting an oil filler pipe assembled using SUS436 as a material to electrostatic coating. Alternatively, Patent Literature 3 discloses a technology of subjecting a stainless steel oil filler pipe to coating for chipping resistance and assuring a sufficient rust prevention property even when the oil filler pipe is subjected to chipping. However, these technologies require higher cost for coating than electrodeposition coating. Meanwhile, an inside of a gap cannot be coated, and therefore there is no guarantee that a sufficient rust prevention effect is obtained in the gap.
Patent Literature 4 discloses a technology of providing a projection on a gap-forming member and controlling an opening amount of the gap to 0.2 mm or more for covering an inside of the gap by electrodeposition coating.
Additionally, a rust prevention method other than coating is also proposed. For example, Patent Literature 5 discloses a technology of allowing for sacrificial protection by disposing a zinc sacrificial anode in a gap or a portion in which a passive state film is impaired by welding, brazing, plastic working, or the like in assembling a stainless steel oil filler pipe. However, it is troublesome and takes time to dispose zinc at all the portions that are at risk of corrosion. As described in Non-patent Literature 1, zinc is consumed easily in a chloride environment, and thus a large amount of zinc is disadvantageously required. In Patent Literature 6, a galvanized steel plate is used for an inlet pipe to eliminate a gap by filling the gap with melted zinc. However, as described above, zinc is consumed very easily in a salt damage-resistant environment. Further, since an oil inlet enters an inlet pipe, melted zinc is likely to invade the inlet pipe to be reacted with water, so that a corrosion product such as zinc hydroxide may be formed to cause clogging of fuel injection equipment.